In conventional systems the Random Access (RA) procedure is the first procedure where the radio network of a wireless communication system knows that a User Device (UD) is trying to establish a connection to the network. At this stage the UD does not have any resource or channel available to inform the network about its desire to connect, so the UD will send its request over a shared medium. There might be many other UDs in the same area sending the same request, in which case there is also a possibility of collisions among the requests coming from different UDs. Such RA procedure is called contention based RA procedure. After a successful RA procedure, the UD is able to start data transmissions with the connected Access Node (AN). The RA procedure for Long Term Evolution (LTE) is mainly specified in 3GPP TS 36.321 and in TS 36.213.
LTE uses the RA procedure for a UD (or User Equipment (UE) in LTE) to make a connection to an AN of the radio network and to obtain the timing advance for the Uplink (UL). Additionally, the RA handles collisions that might happen when two or more UDs are trying to make a connection at the same time. In conventional systems, after each handover decision, the UD performs the connection establishment to the new AN through the RA procedure. The RA is started with a preamble message from the UD. If the AN can decode the preamble successfully, it sends the RA response to the UD, which contains the UL timing advance, a temporary Cell Radio Network Temporary Identifier (C-RNTI) value and a UL grant for Msg3 (the 3GPP term for first scheduled UL transmission in contention based RA procedure), which is to be used for Radio Resource Control (RRC) connection establishment.
Another aspect is the mobility of a UD which is already in connected state. In existing legacy systems the handover decisions are based on measurements made by the UDs during time periods called measurement gaps. The locations of the measurement gaps are specified by network specified parameters. Currently, e.g. in LTE systems, mobility and RA procedure issues are solved for large cells, where UNs are not constantly making handovers from one cell to another. Therefore in LTE, the UD keeps track of the macro-cell ANs and decide which one of them provides the best signal. However, when a handover takes place, the UD is required to establish connection to the new AN through the RA procedure.